Add uint32 cast to unix.SELINUX_MAGIC
Add uint32 cast to unix.SELINUX_MAGIC

Otherwise this value will be treated as `int32`, which results in an
build error in 386 environments:

```
selinux_linux.go:118:14: constant 4185718668 overflows int32
```

Signed-off-by: Sascha Grunert <70ab469ddb2ac3e35f32ed7c2fd1cca514b2e879@suse.com>

opencontainers/selinux